THE US NRCS ACCIDENT SEQUENCE PRECURSOR PROGRAM - AN OVERVIEW AND DEVELOPMENT OF A BAYESIAN-APPROACH TO ESTIMATE CORE DAMAGE FREQUENCY USING PRECURSOR INFORMATION

This paper presents an overview of the US Nuclear Regulatory Commissio n's (NRC) Accident Sequence Precursor Program. The objectives of the p rogram are delineated: the screening, review and analysis methods for events involving an accident initiator are reviewed. The quantitative calculations are illustrated using an ASP event. A Bayesian framework for using ASP information to estimate annual core damage frequencies i s then introduced. This method uses both failure and success informati on to estimate the core damage frequency. In addition, an alternative procedure is proposed to estimate the frequency of severe core damage accidents. The estimated occurrence rate is then used to define a retr ospective accident sequence precursor measure of closeness to a core d amage accident. The retrospective measure is the a posteriori probabil ity of at least one core damage accident during a given period of time . Expressions for both the conditional and unconditional severe core d amage probabilities are developed. The paper also shows that the sum o f conditional severe core damage probabilities overestimates the rate at which severe core damage events occur.